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[zarkfoo]

If I have my GPS receiver mounted to the top of a train, and the train goes around the same track over and over (train is traveling a max of 20MPH)... I would think the GPS coordinates would be identically the same with every complete trip... but that doesn't happen.  Is there a way to get the same GPS coordinates each time?  Also the total number of GPS coordinates read for a complete trip varies... one time it may be a total of 1000, then the next it may be 1085. I've tried enabling and disabling Static Nav and WAAS, but didn't solve my problem.

Any help to point me in the right direction would be greatly appreciated!!

[GS_TECH01]

Hello zarkfoo,

Welcome to the forums! Just a couple of questions about this:

What is the model number of your device?
Are you using the device with a third-party software or with the program that came with it? Please provide details on this.
If the program is running on a computer, what is the operating system and version?

How about the testing environment? Does it always have access to a clear, open sky?
Since it's in a moving train, does it go through a tunnel or to an area where there are tall trees around?

[zarkfoo]

I'm using the MR-350P Bulkhead connected to the serial port of a Windows XP SP2 Dell notebook. I'm a software developer and have written an app that reads the Comm port and records all $GPRMC message to a file.  I've also used the GpsInfo and SiRFDemo software with the same results. The environment is not always open sky... there are heavy wooded areas the train passes through... but even in the woods I continue getting $GPRMC messages.

[GS_TECH01]

Alright, thanks for the info.

The ideal environment for our GPS Receivers, or for any GPS devices out there, is that they should have clear access to an open sky, with no tall trees or buildings around. You can find more info about this in the following forum postings:
http://www.usglobalsat.com/forum/index.php?topic=3872.msg10572#msg10572
http://www.usglobalsat.com/forum/index.php?topic=3780.msg9921#msg9921

Whether the device is able to get a good GPS fix or not, you would still continually see the RMC sentences. In other words, if the device is not able to acquire a fix, then, you would get invalid RMC data. If it's able to acquire a fix, then, you would get a valid, accurate RMC data.

[zarkfoo]

In my app I only accept and log valid RMC data (Status byte = A)... so whether in open sky or the woods I'm constantly receiving valid RMC messages.

[GS_TECH01]

Just a few more questions:

For every train trip, does it travel at the same time everytime?
Also, does it start and arrive to its destination the same time for every other trips?

Since the default output rate is every 1 second, the device continually outputs sentences every second whether the train is moving or not. You can possibly check the timestamp to confirm this.
Because we're not familiar with your app, perhaps, you can modify it or make some changes if there's any filtering scheme. Other than that, you can verify with GPSInfo (http://www.usglobalsat.com/store/downloads/GPSInfo.zip) and see that it's outputting data every one second.

[zarkfoo]

First of all... thanks USG_TECH10 for hanging with me on this.

Every trip takes around 11 minutes... so the goal is to travel at the same time which means probably +- 20 seconds each trip.  Each trip starts and ends at the same spot every time.

In my app I capture not only just valid GPRMC data, but I also log all GPS data as well.  Below are examples from back to back runs which I'm only including the first 5 seconds from each run. You will notice each run begins with "**** 801 ****" which obviously does not come from the GPS receiver.  This comes from an RFID tag that is fix-mounted to the railroad tie and the RFID antenna is fix-mounted to the bottom of the train.  In my app I open a different Comm port so I can read both the GPS data and RFID data from their respective Comm ports as they are received. So my app will start logging GPS data whenver the "801" RFID tag is read... and that's how I know that logging GPS data starts at the same identical spot each time.

Run 1
*********************************** 801 ***********************************
21:12:37 - $GPGGA,031139.000,3623.1137,N,09508.4042,W,2,11,0.7,228.1,M,-28.1,M,2.8,0000*4E
21:12:37 - $GPGSA,A,3,31,22,14,32,25,11,18,30,20,12,01,,1.4,0.7,1.2*38
21:12:37 - $GPRMC,031139.000,A,3623.1137,N,09508.4042,W,4.75,282.33,241112,,*15
21:12:38 - $GPGGA,031140.000,3623.1140,N,09508.4058,W,2,11,0.7,228.1,M,-28.1,M,0.8,0000*49
21:12:38 - $GPGSA,A,3,31,22,14,32,25,11,18,30,20,12,01,,1.4,0.7,1.2*38
21:12:38 - $GPRMC,031140.000,A,3623.1140,N,09508.4058,W,4.70,284.04,241112,,*17
21:12:39 - $GPGGA,031141.000,3623.1143,N,09508.4074,W,2,11,0.7,228.1,M,-28.1,M,0.8,0000*45
21:12:39 - $GPGSA,A,3,31,22,14,32,25,11,18,30,20,12,01,,1.4,0.7,1.2*38
21:12:39 - $GPGSV,3,1,12,31,75,216,51,14,47,052,45,22,43,134,45,32,41,314,37*7F
21:12:39 - $GPGSV,3,2,12,25,31,074,39,01,30,289,38,11,21,265,42,30,13,174,42*7C
21:12:39 - $GPGSV,3,3,12,12,12,042,34,20,10,313,37,18,05,133,23,51,45,199,54*74
21:12:39 - $GPRMC,031141.000,A,3623.1143,N,09508.4074,W,4.69,284.39,241112,,*1D
21:12:40 - $GPGGA,031142.000,3623.1147,N,09508.4090,W,2,11,0.7,228.1,M,-28.1,M,0.8,0000*48
21:12:40 - $GPGSA,A,3,31,22,14,32,25,11,18,30,20,12,01,,1.4,0.7,1.2*38
21:12:40 - $GPRMC,031142.000,A,3623.1147,N,09508.4090,W,4.66,283.95,241112,,*1E
21:12:41 - $GPGGA,031143.000,3623.1150,N,09508.4106,W,2,11,0.7,228.1,M,-28.1,M,0.8,0000*41
21:12:41 - $GPGSA,A,3,31,22,14,32,25,11,18,30,20,12,01,,1.4,0.7,1.2*38
21:12:41 - $GPRMC,031143.000,A,3623.1150,N,09508.4106,W,4.69,283.78,241112,,*1B
21:12:42 - $GPGGA,031144.000,3623.1153,N,09508.4121,W,2,11,0.7,228.1,M,-28.1,M,1.8,0000*41
21:12:42 - $GPGSA,A,3,31,22,14,32,25,11,18,30,20,12,01,,1.4,0.7,1.2*38
21:12:42 - $GPRMC,031144.000,A,3623.1153,N,09508.4121,W,4.64,283.31,241112,,*1A
21:12:43 - $GPGGA,031145.000,3623.1157,N,09508.4137,W,2,11,0.7,228.1,M,-28.1,M,2.8,0000*40
21:12:43 - $GPGSA,A,3,31,22,14,32,25,11,18,30,20,12,01,,1.4,0.7,1.2*38
21:12:43 - $GPRMC,031145.000,A,3623.1157,N,09508.4137,W,4.67,283.34,241112,,*1E
21:12:44 - $GPGGA,031146.000,3623.1160,N,09508.4153,W,2,11,0.7,228.1,M,-28.1,M,3.8,0000*44
21:12:44 - $GPGSA,A,3,31,22,14,32,25,11,18,30,20,12,01,,1.4,0.7,1.2*38
21:12:44 - $GPGSV,3,1,12,31,76,218,51,14,47,053,46,32,42,314,40,22,42,134,45*72
21:12:44 - $GPGSV,3,2,12,25,31,073,40,01,30,288,38,11,21,264,42,30,14,174,42*72
21:12:44 - $GPGSV,3,3,12,12,11,041,30,20,11,313,37,18,05,133,32,51,45,199,54*71
21:12:44 - $GPRMC,031146.000,A,3623.1160,N,09508.4153,W,4.74,283.04,241112,,*1A
21:12:45 - $GPGGA,031147.000,3623.1163,N,09508.4169,W,2,11,0.7,228.2,M,-28.1,M,0.8,0000*4F
21:12:45 - $GPGSA,A,3,31,22,14,32,25,11,18,30,20,12,01,,1.4,0.7,1.2*38
21:12:45 - $GPRMC,031147.000,A,3623.1163,N,09508.4169,W,4.79,283.85,241112,,*15

Run 2
*********************************** 801 ***********************************
21:36:29 - $GPRMC,033531.000,A,3623.1141,N,09508.4050,W,4.50,284.06,241112,,*1E
21:36:30 - $GPGGA,033532.000,3623.1144,N,09508.4065,W,2,10,0.8,228.7,M,-28.1,M,0.8,0000*48
21:36:30 - $GPGSA,A,3,31,22,14,32,25,11,20,30,12,01,,,1.5,0.8,1.3*3E
21:36:30 - $GPRMC,033532.000,A,3623.1144,N,09508.4065,W,4.59,283.96,241112,,*19
21:36:31 - $GPGGA,033533.000,3623.1147,N,09508.4081,W,2,10,0.8,228.7,M,-28.1,M,1.8,0000*41
21:36:31 - $GPGSA,A,3,31,22,14,32,25,11,20,30,12,01,,,1.5,0.8,1.3*3E
21:36:31 - $GPRMC,033533.000,A,3623.1147,N,09508.4081,W,4.66,283.70,241112,,*15
21:36:32 - $GPGGA,033534.000,3623.1150,N,09508.4097,W,2,10,0.8,228.7,M,-28.1,M,2.8,0000*44
21:36:32 - $GPGSA,A,3,31,22,14,32,25,11,20,30,12,01,,,1.5,0.8,1.3*3E
21:36:32 - $GPRMC,033534.000,A,3623.1150,N,09508.4097,W,4.73,284.79,241112,,*19
21:36:33 - $GPGGA,033535.000,3623.1154,N,09508.4113,W,2,10,0.8,228.6,M,-28.1,M,0.8,0000*4F
21:36:33 - $GPGSA,A,3,31,22,14,32,25,11,20,30,12,01,,,1.5,0.8,1.3*3E
21:36:33 - $GPRMC,033535.000,A,3623.1154,N,09508.4113,W,4.80,284.78,241112,,*1C
21:36:34 - $GPGGA,033536.000,3623.1157,N,09508.4129,W,2,10,0.8,228.6,M,-28.1,M,0.8,0000*46
21:36:34 - $GPGSA,A,3,31,22,14,32,25,11,20,30,12,01,,,1.5,0.8,1.3*3E
21:36:34 - $GPGSV,3,1,12,31,85,278,52,32,52,313,48,14,40,064,45,22,32,140,39*7A
21:36:34 - $GPGSV,3,2,12,01,29,276,39,25,29,061,42,30,23,171,40,20,20,314,37*76
21:36:34 - $GPGSV,3,3,12,11,17,255,41,12,06,034,27,23,03,279,33,51,45,199,49*7B
21:36:34 - $GPRMC,033536.000,A,3623.1157,N,09508.4129,W,4.84,283.80,241112,,*11
21:36:35 - $GPGGA,033537.000,3623.1160,N,09508.4146,W,2,10,0.8,228.6,M,-28.1,M,0.8,0000*4A
21:36:35 - $GPGSA,A,3,31,22,14,32,25,11,20,30,12,01,,,1.5,0.8,1.3*3E
21:36:35 - $GPRMC,033537.000,A,3623.1160,N,09508.4146,W,4.88,284.25,241112,,*19

[GS_TECH01]

Sorry for the late response.

Got it. Thanks for the answers. So, it doesn't travel at the same time everytime. That's why you're getting different number of coordinates for each trip.

After looking at the log you provided, it does outputs the RMC data every one second. I could not find anything alarming or strange as well.

Moreover, I would not consider this an issue since it outputs accurate data properly and the device is able to get a good GPS signal.  Thanks.